1. Field of the Invention
The present invention relates to an improvement in a contact type image sensor for use in an image reading apparatus or the like, and more specifically, to elimination of the effect caused by spike noises generated from a switching circuit included in the image reading apparatus.
2. Description of the Related Art
A contact type image sensor comprises a light receiving array made up of a plurality of light receiving elements and a circuit for performing a scanning over the light receiving element array, and is arranged to read an original manuscript closely disposed to the light receiving element array (whose length corresponds to the width of the original manuscript) or through such an optical system as an optical fiber array or a lens array in a 1:1 image relation. With this type of sensor, the length of the light path can be shorter than that using a MOS image senor or a CCD image sensor. As a result, an apparatus employing contact type image sensor can be remarkably reduced in size.
FIG. 1 is plan view showing a featured part of a contact type image sensor 10 and FIG. 2 is a cross-sectional view taken along line A--A' in FIG. 1. In the illustrated sensor 10, a plurality of electrodes Tl to Tn are formed as spaced by a proper distance therebetween on an insulating substrate 11, a photoconductive thin film 12 is formed on parts of the electrodes Tl to Tn, and then an electrically conductive transparent thin film 13 is formed on the thin film 12. That is, the electrodes Tl to Tn and films 12 and 13 form a laminated layer structure. The electrodes Tl to Tn are connected at the other ends (that is, opposite to the thin films 12 and 13) to a switching circuit 14 through bonding wires Wl to Wn.
The insulating substrate 11 may be made of glass, ceramic, a silicone wafer, or may comprise a metallic plate having insulated surface. The electrodes Tl to Tn may be made of chromium, molybdenum, tungsten, tantalum or nickel. The photoconductive thin layer 12 may be made of amorphous silicon hydride, Se-As-Te, CdS, CdSe and so on. The electrically conductive film 13 may be made of SnO.sub.2, In.sub.2 O.sub.5 or the like. In FIGS. 1 and 2, the areas intersected by the lower electrodes Tl to Tn and the electrically-conductive upper film 13 work as light receiving elements Ll to Ln.
FIG. 3 shows a circuit diagram of the image sensor 10 of FIGS. 1 and 2, in which the light receiving elements Ll to Ln are equivalently expressed by parallel circuits of photodiodes PDl to PDn and capacitors PCl to PCn, respectively. When an image on an original is optically formed on the light receiving elements Ll to Ln, light currents corresponding to the intensity of light received at the associated photodiodes will flow through the photodiodes PDl to PDn respectively. Electric charges corresponding to the magnitudes of the light currents will be stored in the associated capacities Cl to Cn which are respective sums of the stray capacitances of the electrodes Tl to Tn, the junction capacitances between the drain and source of the MOS transistors Sl to Sn, and so on. When the MOS transistors Sl to Sn are sequentially turned ON at intervals of a predetermined constant time under the control of a shift register 15, that is, when the transistors Sl to Sn perform a scanning operation, the charges stored in the respective capacities Cl to Cn will be discharged through a load resistor 17 connected via a signal output line 16 to the capacities Cl to Cn. A current flowing through the load resistor 17 is derived out as a signal indicative of the original document image information. A power source 18 is used to bias the light receiving elements Ll to Ln.
With the contact type image sensor 10, the electric charges stored in the capacities Cl to Cn during the scanning of the MOS transistors Sl to Sn are discharged to use them as an output signal. For this reason, spike noises generated at the time of turning ON the MOS transistors Sl to Sn will appear on the signal output line 16 as noise charges. Assume that the amount of signal charge stored in the capacity Cl is 1 (pC), the gate voltage of the MOS transistor Sl is 5 (V) and the junction capacitance between the gate and source of the transistor is 1 (pF). Then, the amount of noise charge appearing on the signal output line 16 due to the spike noise will be 5 (pC) (=(pF).times.5 (V)), that is larger than the signal charge (1 (pC)). Further, the time necessary to discharge the signal charge of each of the capacities Cl to Cn is 1 (.mu.sec) or shorter and thus the current flowing through the resistor 17 is very small. Therefore, the image sensor requires a noise cancellation circuit and a high speed, high gain amplifier. Thus, an apparatus incorporating the contact type image sensor cannot be reduced in size.
Further, since the electrodes Tl to Tn are arranged in parallel with each other on the substrate l with a high density, the capacitance between the adjacent electrodes is not negligible. If the width and length of each of the electrodes Tl to Tn are respectively 70 (.mu.m) and 2 (cm) and the spacing between the adjacent electrodes is 125 (.mu.m), then the capacitance between the adjacent electrodes will be between 1 and 3 (pF). FIG. 4 is a circuit diagram of a contact type image sensor in which such electrode-to-electrode capacitance is taken into account. However, this sensor has also a problem that, when the MOS transistor Sl is turned ON to discharge the signal charge of the capacity Cl, the capacities C.sub.2 and C.sub.3 are also discharged through a capacitance C' between the electrodes, whereby crosstalk takes place in the signal charge.